1. Field of the Invention
The present invention relates to a projector and projection method, which can form an image with high resolution, and more particularly relates to a projector and projection method that can be used effectively to define a circuit pattern in a lithography process in the field of semiconductor integrated circuit technologies.
2. Description of the Related Art
Projectors for performing a drawing operation with a spatial light modulator such as a micromirror array have been proposed. A system using Digital Micromirror Device (DMD; a product name) is well known as such a projector. A system of that type is expected to work in not only display applications but also various other fields such as semiconductor lithography and photo printing.
Japanese Laid-Open Publication No. 10-112579 discloses a technique of applying the DMD to a semiconductor lithography process. According to that technique, no photomask is used for exposure purposes but an exposure process is carried out by presenting an image representing a circuit pattern on the DMD and then projecting the light reflected from the DMD onto a photoresist.
In a normal DMD, each micromirror changes its tilt angles in two steps. However, in the spatial light modulator disclosed by Peter Duerr, et al. in “Characterization of Spatial Light Modulators for Micro Lithography”, Proc. of SPIE Vol. 4985, pp. 211–221 (Jan. 28–29, 2003), the tilt angle of each micromirror is changed in multiple steps with a multi-bit signal. Duerr at al. also disclosed that a lithography process should be carried out with a micromirror array driven by a multi-bit control. More specifically, according to Duerr's technique, micromirrors associated dark regions on the projection plane are given a maximum tilt, while micromirrors associated with bright regions on the projection plane are given a minimum tilt. Meanwhile, other micromirrors associated with the boundary regions between those dark and bright regions are given an intermediate tilt. And if the intermediate tilt angle is changed, then the boundary between the dark and bright regions will shift according to Duerr et al.
However, as for the conventional arrangement described above, it is difficult to apply a high-resolution masking technique such as a phase shifting method, and therefore, the size of the pattern being drawn on the projection plane cannot be reduced beyond a certain limit.
For example, when the DMD is used as micromirrors as disclosed in Japanese Laid-Open Publication No. 10-112579, each of those micromirrors changes the quantity of transmitted light digitally (i.e., by switching between ON and OFF states) and works just like a normal mask with an opening. A pair of diffracted light rays, coming from two adjacent bright regions, has the same phase and interferes with each other. As a result, it is difficult to separate the images of the two bright regions from each other on the projection plane.
Essentially the same statement applies to the analog control of mirror tilt angles as disclosed by Duerr et al. That is to say, each mirror can control nothing but its tilt angle. Accordingly, whenever a half of a mirror surface is raised, the other half is lowered so that the average displacement of the overall mirror surface is always equal to zero. This means that the reflected light, modulated by such a mirror, has an average phase shift of zero. Thus, a pair of diffracted light rays, coming from two adjacent bright regions, also has the same phase and interferes with each other. Consequently, it is difficult to separate the images of the two bright regions from each other on the projection plane, too.
For that reason, each of the two conventional techniques described above results in decreased resolution as compared with a situation where a phase shift mask is used.